There are several advantages by using powder metallurgical methods for producing structural parts compared with conventional matching processes of fully dense steel. Thus the energy consumption is much lower and the material utilisation is much higher. Another important factor in favour of the powder metallurgical route is that components with net shape or near net shape can be produced directly after the sintering process without costly shaping such as turning, milling boring or grinding. However, normally a full dense steel material has superior mechanical properties compared with powder metallurgy (PM) components. Therefore, the goal has been to increase the density of PM components in order to reach values as close as possible to the density value of fully dense steel. At relatively high sintered densities the occurrence of porosity in a powder metallurgical body has mainly negatively influence of the dynamic mechanical properties, the fatigue properties. Additives and processing routes giving small pore sizes and round pores in the sintered body may diminish the negative effects of the porosity.
One area of future growth in the utilization of powder metal parts is in the automotive industry. Of special interest within this field is the use of powder metal parts in more demanding applications, such as power transmission applications, for example, gear wheels. Presently the most commonly used material for producing gear wheels for demanding automotive applications are based on wrought steel of the type 16MnCr5, 15CrNi6, or SAE 8620.
Problems with gear wheels formed by the powder metal process are that powder metal gear wheels have reduced bending fatigue strength in the tooth root region of the gear wheel, and low contact fatigue strength on the tooth flank compared with gears machined from bar stock or forgings. These problems may be reduced by plastic deformation of the surface of the tooth root and flank region through a process commonly known as surface densification. Products which can be used for these demanding applications are described, for example, in U.S. Pat. Nos. 5,711,187, 5,540,883, 5,552,109, 5,729,822, 6,171,546 and U.S. Patent Application Publication US 2004/0177719.
The U.S. Pat. No. 5,711,187 (1990) is particularly concerned with the degree of surface hardness, which is necessary in order to produce gear wheels, which are sufficiently wear resistant for use in heavy-duty applications. According to this patent the surface hardness or densification should be in the range of 90 to 100 percent of full theoretical density to a depth of at least 380 microns and up to 1,000 microns. No specific details are disclosed concerning the production process but it is stated that admixed powders are preferred as they have the advantage of being more compressible, enabling higher densities to be reached at the compaction stage. Furthermore it is stated that the admixed powders should include in addition to iron and 0.2% by weight of graphite, 0.5% by weight of molybdenum, chromium and manganese, respectively.
A method similar to that described in the U.S. Pat. No. 5,711,187 is disclosed in U.S. Pat. No. 5,540,883 (1994). According to the U.S. Pat. No. 5,540,883, bearing surfaces from powder metal blanks are produced by blending carbon and ferro alloys and lubricant with compressible elemental iron powder, pressing the blending mixture to form the powder metal blank, high temperature sintering the blank in a reducing atmosphere, compressing the powder metal blanks so as to produce a densified layer having a bearing surface, and then heat treating the densified layer. The sintered powder metal article should have a composition, by weight percent, of 0.5 to 2.0% chromium, 0 and 1.0% molybdenum, 0.1 and 0.6% carbon, with a balance of iron and trace impurities. Broad ranges as regards compaction pressures are mentioned. Thus it is stated that the compaction may be performed at pressures between 25 and 50 ton per square inch (about 390-770 MPa).
The U.S. Pat. No. 5,552,109 (1995) concerns a process of forming a sintered article having high density. The patent is particularly concerned with the production of connecting rods. As in the U.S. Pat. No. 5,711,187, no specific details concerning the production process are disclosed in the U.S. Pat. No. 5,552,109, but it is stated that the powder should be a pre-alloyed iron based powder, that the compacting should be performed in a single step, that the compaction pressures may vary between 25 and 50 ton per square inch (390-770 MPa) to green densities between 6.8 and 7.1 g/cm3 and that the that the sintering should be performed at high temperature, particularly between 1270 and 1350° C. It is stated that sintered products having a density greater than 7.4 g/cm3 are obtained and it is thus obvious that the high sintered density is a result of the high temperature sintering.
In U.S. Pat. No. 5,729,822 (1996), a powder metal gear wheel having a core density of at least 7.3 g/cm3 and a hardened carburized surface is disclosed. The powders recommended are the same as in the U.S. Pat. Nos. 5,711,187 and 5,540,883, which discuss mixtures obtained by blending carbon, ferro alloys and lubricant with compressible powder of elemental iron. In order to obtain high sintered core density the patent mentions warm pressing; double pressing, double sintering; high density forming as disclosed in the U.S. Pat. No. 5,754,937; the use of die wall lubrication, instead of admixed lubricants during powder compaction; and rotary forming after sintering. Compacting pressures of around 40 tons per square inch (620 MPa) are typically employed.
Also the U.S. Pat. No. 6,171,546 discloses a method for obtaining a densified surface. According to this patent the surface densification is obtained by rolling or, preferably, by shot peening of a green body of an iron-based powder. From this patent it can be concluded that the most interesting results are obtained if a pre-sintering step is performed before the final densification and sintering operations. According to this patent the sintering can be performed at 1120° C., i.e. at conventional sintering temperatures, but as two sintering steps are recommended the energy consumption will be quite considerable.
The U.S. Patent Application Publication US 2004/0177719 describes a method of forming powder metal materials and parts, such as gears and sprockets, having surface regions that are uniformly densified to full density to depth ranging from 0.001 inches to 0.040 inches, and core regions that can have at least 92 percent theoretical density and further can have essentially full density, i.e., 98% and above.
The surface densification of sintered PM steels is discussed in, e.g., the Technical Paper Series 8202 234, (International Congress & Exposition, Detroit, Mich., Feb. 22-26, 1982). In this paper a study of surface rolling of sintered gears is reported. Fe—Cu—C and Ni—Mo alloyed materials were used for the study. The paper reveals the results from basic research on the surface rolling of sintered parts and the application of it to sintered gears. The basic studies includes surface rolling with different diameters of the rolls, best results in terms of strength were achieved with smaller roll diameter, lesser reduction per pass and large total reduction. As an example for a Fe—Cu—C material a densification of 90% of theoretical density was achieved with a roll of 30 mm diameter to a depth of 1.1 mm. The same level of densification was achieved to a depth of about 0.65 mm for a 7.5 mm diameter roll. The small diameter roll however was able to increase the densification to about full density at the surface whereas the large diameter roll increased the density to about 96% at the surface. The surface rolling technique was applied to sintered oil-pumps gears and sintered crankshaft gears. In an article in Modern Developments in Powder Metallurgy, Volume 16, p 33-48 1984 (from the International PM Conference, Jun. 17-22,1984 Toronto Canada), the authors have investigated the influence of shot-peening, carbonitriding and combinations thereof on the endurance limit of sintered Fe+1.5% Cu and Fe+2% Cu+2.5% Ni alloys. The densities reported of these alloys were 7.1 and 7.4 g/cm3. Both a theoretical evaluation of the surface rolling process and a bending fatigue testing of surface rolled parts is published in an article in Horizon of Powder Metallurgy part 1, p. 403-406, Proceedings of the 1986 (International Powder Metallurgy Conference and Exhibition DOusseldorf, 7-11 Jul. 1986).
According to the prior art above many different routes have been suggested in order to reach a high sintered density of a powder metallurgical component. However the suggested processes all includes steps adding additional costs such as warm compaction, double pressing-double sintering, die wall lubrication, high temperature sintering etc. Furthermore, for high load applications such as gear wheels, dynamic mechanical properties such as bending fatigue strength and contact fatigue strength needs to reach the same level as gear wheels produced from full dense steel. A simple and cost effective method for the preparation of gear wheels and similar products having dynamic mechanical properties equal to wrought gear wheels, would thus be attractive.